1. Field of the Invention
This invention relates to a thermal image transfer recording medium which can yield images with high density with a minimized decrease in the image density even when it is used repeatedly.
2. Discussion of Background
Recording apparatus, such as a printer and a facsimile apparatus, using a thermal image transfer recording method, is now widely used. This is because the recording apparatus of this type is relatively small in size and can be produced inexpensively, and the maintenance is simple.
In a conventional thermal image transfer recording medium for use with the thermal image transfer recording apparatus, a single ink layer is merely formed on a support. When such a recording medium is used for printing images, the portions of the ink layer heated by a thermal head are completely transferred to an image receiving sheet at only one-time printing, so that the recording medium can be used only once, and can never be used repeatedly. The conventional recording medium is thus disadvantageous from the viewpoint of running cost.
In order to overcome the above drawback in the prior art, there have been proposed the following methods:
(1) A microporous ink layer containing a thermofusible ink is formed on a support so that the ink can gradually ooze out from the ink layer as disclosed in Japanese Laid-Open Patent Applications 54-68253 and 55-105579;
(2) A porous film is provided on an ink layer formed on a support so that the amount of an ink which oozes out from the ink layer can be controlled as disclosed in Japanese Laid-Open Patent Application 58-212993; and
(3) An adhesive layer is interposed between an ink layer and a support so that an ink in the ink layer can be gradually exfoliated in the form of a thin layer from the ink layer when images are printed as disclosed in Japanese Laid-Open Patent Applications 60-127191 and 60-127192.
However, the above three methods have shortcomings as described below.
When the above method (1) is employed, the ink cannot sufficiently ooze out after repeated use of the recording medium. As a result, the density of printed images gradually decreases as the printing operation is repeated.
In the method (2), the mechanical strength of the porous film is decreased if the size of each pore is increased in order to increase the image density, and thus the ink layer tends to peel off the support, together with the porous film.
As for the method (3), the amount of the ink which peels off the ink layer cannot be controlled uniformly in the course of image printing.
Furthermore, most of the conventional methods have been developed in such a fashion as to be suitable for use with a serial thermal head in a recording apparatus such as a word processor. Therefore, when those methods are applied to a line thermal head for use in a recording apparatus such as a facsimile apparatus and a bar code printer, problems such as the exfoliation of an ink layer, and the decrease of image density are inevitable because the time elapsed before an image transfer sheet is separated from the image transfer recording medium is relatively long after the image transfer sheet is brought into contact with the image receiving sheet under application of heat thereto.
In addition, in a thermofusible ink prepared by a conventional method, the ink-dispersed system itself tends to be destroyed by the heat applied thereto by a thermal head in the course of repeated printing. As a result, the optical density of the image printed on an image receiving sheet by the ink is no longer high enough for us in practice.
Under these circumstances, there is a demand for a thermal image transfer recording medium which is suitable for use with a line thermal head and can yield images with high image density with a minimum decrease in the image density even when it is used repeatedly.